Characterization of Disease Resistance of Carica papaya to Phytophthora

Blights and root rots caused by Phytophthora spp., are one of the more difficult diseases to control in a number of crop species. Papaya (Carica papaya L.), has limited resistance to root and stem rot, caused by Phytophthora palmivora. Since host resistance is often lacking, use of fungicides is the primary method used by growers to control blights and root rots. In the previous study, we reported that systemic acquired resistance (SAR)in papaya can be activated by both elicitor and chemical Benzo (1,2,3)thiadiazole-7-carbothioic acid S-methyl ester (BTH). In order to understand the induced resistance mechanism of papaya against Phytophthora palmivora, we produced transgenic papaya to over-express the key regulatory gene, NPR1, in SAR pathway. Two separate transformations, using one construct containing the AtNPR1 and the other, an NPR1 gene homolog from papaya (ppyNPR1), were carried out using biolistic delivery system. The constitutive CaMV35S promoter was used to drive the desired genes and NPTII gene as a selectable marker. Molecular analysis was carried out to detect the presence of the transgene in selected plants. Transgenic plants were challenged with Phytophthora palmivora, to determine the change in the level of resistance against the pathogen. The result of leaf disc bioassays showed that smaller necrotic lesions developed on transgenic lines compared to wild-type plants, indicating the resistance in papaya has been improved. Our results indicate that pathogenesis-related (PR)and NPR1 genes are involved in disease resistance of papaya and that induction of SAR has a potential for being used for crop protection.